Holographic code division multiple access

ABSTRACT

A technique for all-optical Code Division Multiple Access (CDMA) system based on optical holography is disclosed. In this technique the energy of an incoming information light signal is spread over a spatial domain by a two-dimensional spatial encoder which includes a mask having regions of first and second transmission characteristics corresponding to the unique code assigned to a particular source. Subsequent decoding, which is accomplished by an optical matched filter through the use of a hologram, spatially despreads the energy of the information light signal and produces a focused light beam which serves as input to a code division detector.

FIELD OF THE INVENTION

This invention relates generally to optical fiber communication systemsand, in particular, to code division multiple access systems wherein theinformation content communicated between each source/receiver pair isdecoupled from the encoding/decoding operations.

BACKGROUND OF THE INVENTION

The proliferation of fiber-optic cables and the ever increasing demandfor new broadband services is moving future telecommunication networkstoward all-optical networks. By design, all-optical networks perform keysignal processing such as switching, multiplexing, demultiplexing,amplification, and correlation, with optical systems and avoidelectrical-to-optical and optical-to-electrical conversions. Opticalsystems or optical signal processing should alleviate the predictedbottleneck that could occur with complex high-speed electronic switches,multiplexers, demultiplexers, and so forth, because all-opticaltechniques are potentially much faster than electrical signalprocessing. Several new classes of optical networks are emerging. Inparticular, code division multiple access (CDMA) networks using opticalsignal processing technique have been recently introduced. For example,the special issue on "Optical Multiaccess", as published in the IEEENetwork Magazine, vol. 3, no. 2, March 1989 provides an overview of thisemerging field.

In a typical CDMA system, multiaccess is achieved by assigningdifferent, minimally interfering code sequences to different user pairs.Users then communicate by imprinting their message bits upon their ownunique code, which they transmit asynchronously (with respect to theother transmitters) over a common channel. A matched filter at thereceiver end ensures that message bits are detected only when they areimprinted on the proper code sequence. This approach to multiaccessallows transmission without delay and handles multiaccess interferenceas an integral part of the scheme.

Processing gain (PG) for CDMA techniques is a critical parameter whichmay be used to judge the relative merits of CDMA systems. Processinggain is broadly given by the ratio of total transmitted bandwidth toinformation bandwidth of a transmitter. The value of PG establishes anupper bound on the number of users/transmitters that can besimultaneously active on a given CDMA system. Presently known CDMAtechniques such as spread-spectrum and spread-time (which will bediscussed in detail below) can only incrementally increase the PG sincethe total transmitted bandwidth is usually fixed, implying that theinformation bandwidth must be decreased in order to increase the PG. Alarge reduction in information bandwidth is difficult to achieve forarbitrary information sources.

In addition, with conventional CDMA techniques all transmitters haveessentially the same signal format and data rate. This precludes a mixedmultiuser environment wherein it is desired to transmit analog voice,low rate data, a TV signal, and so forth simultaneously over the CDMAsystem.

SUMMARY OF THE INVENTION

These shortcomings and other limitations are obviated, in accordancewith the present invention, by arranging a CDMA system such that:parameters determining the processing gain are decoupled from both totalbandwidth and information bandwidth; and the signal format andinformation rate of each user is independent of other users.

Broadly, the CDMA system in accordance with the present inventioninterconnects numerous sources to corresponding receivers through anoptical broadcast network. Optical encoders are interposed between thesources and the optical broadcast network, and optical detectors arelocated between the optical broadcast network and the receivers. Eachoptical encoder includes a two-dimensional, spatial encoding mask forencoding a light signal produced by the associated source. The spatialencoding mask is determined from sequences having appropriateautocorrelation and crosscorrelation properties, so that each encodergenerates a corresponding optically encoded signal. The opticalbroadcast network combines the numerous optically encoded signalsproduced by the encoders to generate a composite optical signal composedof all the encoded light signals. Each optical detector is assigned todetect one (or more) of the optically encoded signals, that is, theinformation content of one of the sources, and each optical detectorincludes a holographic decoding mask corresponding to the assignedsource.

The organization and operation of this invention will be understood froma consideration of the detailed description of the illustrativeembodiment, which follows, when taken in conjunction with theaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagram of the all-optical spread-space code divisionmultiple access system in accordance with the present invention;

FIG. 2 illustrates two holographic masks assigned to first and secondsource-receiver pairs;

FIG. 3 lists the array of (0,1) pixels from an exemplary holographicmask;

FIG. 4 depicts an illustrative embodiment of an optical broadcastnetwork used to form a composite light signal;

FIG. 5 is an arrangement for generating unique holograms used in opticalmatched filters; and

FIG. 6 illustrates the placement of collimating lens relative to thehologram of each decoding arrangement to achieve matched filterdetection.

DETAILED DESCRIPTION

To properly place in perspective the inventive aspects of the presentinvention, an overview of the present invention couched in terms relatedto prior art techniques is first presented. This approach also has theadvantage of introducing notation and terminology. Next, a detaileddescription of an illustrative embodiment completes the disclosure.

Overview

The techniques of spread-spectrum and spread-time CDMA spread the energyof the information signal over a wide frequency band or over a long timeperiod, respectively. For an elucidating discussion of spread-spectrumtechniques, the article entitled "Spread Spectrum for CommercialCommunications," by Schilling et al, as published in IEEE CommunicationsMagazine, Vol. 29, No. 4, April 1991 is particularly relevant. Also, thepaper entitled "Spread Time Code Division Multiple Access" by Crespo,Honig, and Salehi, as published in the Globecom Proceedings, December,1991 provides a detailed discussion of the spread-time approach.

In accordance with the present invention, which is referred togenerically as spread-space CDMA, energy of the information signal isspread over a large spatial domain. In general, spreading the energy ofa given information signal and the subsequent despreading of the energyis known as encoding and decoding of the information signal. Therefore,in the spread-spectrum technique, encoding and decoding are obtained intime domain, and in the spread-time technique, encoding and decoding areobtained in frequency domain, whereas in the spread-space technique,encoding and decoding are obtained in spatial domain.

In both spread-spectrum and spread-time CDMA techniques, informationwaveforms (modulating signals) are represented as digital signals, i.e.,they exclude the use of analog signals, and all users have identical bitrate and signal format. However, in spread-space CDMA technique, themodulating signal for each user can take on any form (digital oranalog), any rate, and any shape. For example, in a multiuserenvironment a particular user can be sending analog video while otherusers are sending digitized voice, analog voice, low rate data, and avery high rate data signal. This means that the spread-space CDMAtechnique remains transparent to the form of modulation format of eachuser. This advantage is obtained by transferring the CDMA encoding anddecoding to spatial domain while modulating the information signal inthe time domain.

Processing Gain (PG) for spread-spectrum and spread-time systems isdefined as the ratio of total transmitted bandwidth (basically, afunction of encoding and decoding speed) to the information bandwidth.Processing Gain is the single most important design parameter in anyCDMA system. Its value puts a limit on the number of users that canoperate simultaneously in a CDMA system. To increase the number of usersin a CDMA system from its present value (operating at some bit-errorrate), the PG for that system must increase. There are two ways toincrease the PG. First, by increasing the channel bandwidth (that is,the encoding and decoding speed), or second, by reducing the informationrate. In optical networks where channel bandwidth is not as scarce acommodity as in other systems, one may design all-optical encoders anddecoders that have speeds which are 3 to 4 orders of magnitude fasterthan the information source. But, in CDMA systems such as in radiocellular telephony channel bandwidth is finite and scarce. Thus,reducing the information rate may prove to be the only realisticalternative. The difficulty in increasing PG, thereby increasing thenumber of simultaneous users, with the spread-spectrum and spread-timeCDMA techniques is due to their dependency on the input informationrate. However, the uncoupling between the two domains of signalprocessing, i.e., spatial domain for CDMA encoding and decoding and timedomain for information modulation, contributes to another and a veryimportant feature in spread-space CDMA, namely, the uncoupling of the PGfrom its input information rate. That is, input information rate foreach user can be different while the PG for all the users is the same.Processing Gain in an optical spread-space CDMA, which will be discussedin more detail below, is proportional to the number of pixels in a maskand the value of PG is independent of input information rate or from anymodulation format. The number of distinct pixels in a mask of area A canbe as high as ##EQU1## where λ is the wavelength of the light used inthe system. For A=1 cm² and typical operating wavelengths (1-2 microns)this corresponds to 10⁶ -10⁸ pixels or PG for spread-space CDMA system.Since the PG for typical spread-spectrum or spread-time CDMA techniquesis 10² -10³, then spread-space CDMA can potentially support 4 to 5orders of magnitude more usres. For example, if one to ten percent of PGis taken as the number of users that can be supported by any of the CDMAtechniques, then spread-space CDMA can support as many as tens ofthousands to a few million users where each user can utilize any inputinformation rate and any modulation format.

Illustrative Embodiment

Spread-space CDMA system 100, depicted partly in block diagram form andpartially in component form in FIG. 1, interconnects sources 101, 102, .. . 103 to receivers 111, 112 . . . 113, respectively. (System 100 mayalso be referred to as a Holographic CDMA system for reasons that willbecome apparent as the description proceeds.) Each source 101, 102, or103 can produce either analog or digital signals, may operate at anarbitrary information rate, and need not be compatible with the othersources. Moreover, the information generator included within each source(not explicitly shown), such as a voice signal or a TV camera, may beelectronic so that each source 101, 102, or 103 would include anelectro-optical interface to its corresponding fiber medium. Eachreceiver 111, 112, or 113, which is matched to a corresponding source inthe sense that each receiver is arranged to detect the analog or digitalformat at the incoming information rate, either electronically orelectro-optically depending on the original information generator at thecorresponding source.

Since each source 101, 102, or 103 is arranged with an interface so asto propagate an optical signal representative of the information contentof the source, a monochromatic light signal is propagated onto a fiberoptic medium associated with each source; for example, source 101launches a monochromatic light signal onto fiber 121. Holographicencoding for the monochromatic light signal emanating from fiber 121 isobtained by: (1) collimating the monochromatic light signal withcollimating lens 141; and (2) modulating the collimated monochromaticlight signal emerging from lens 141 with a two-dimensional mask 151having an array of two-dimensional code elements, that is, modulation isobtained by placing mask 151 behind collimating lens 141. Mask 151 has atransmission characteristic which is proportional to a two-dimensionalcode. Exemplary code elements are members the set (0,1), where a 0corresponds to opaque area on mask 151 and a 1 corresponds to atransparent area on mask 151. (Another exemplary set is (+1,-1), where+1 corresponds to transmission with zero phase shift, and -1 correspondsto transmission with a π phase shift). Two typical two-dimensionalcodes, designated s₁ (x,y) and s₂ (x,y), where x and y are spatialcoordinates, are shown in FIG. 2, and illustratively correspond to masks151 and 152, respectively. For each exemplary mask 151 or 152, there are1024 (32×32) pixels, that is, the code length for each mask is 1024. The32×32 pixels array for mask 151 is listed in FIG. 3. It is possible tohave as many as 10⁶ -10⁸ pixels in a 1 cm by 1 cm mask.

The two-dimensional codes for Holographic CDMA can be obtained frombinary sequences of length n, whose autocorrelation is either 1 or##EQU2## by the conventional method discussed in the paper "PseudorandomSequences and Arrays", authored by F. Macwilliams and N. Sloane, andpublished in the Proceedings of the IEEE, Vol. 64, No. 12, pp.1715-1729, December, 1976. The two-dimensional codes as described in thereference generally satisfy the requirements of randomness and haveautocorrelation and crosscorrelation properties that are necessary forthe family of two-dimensional codes used for Holographic CDMA systems.For a pseudorandom array (a two-dimensional code with flatautocorrelation function) with n pixels there are n different arrays,with each array obtained simply by considering each shift of theoriginal array to be a different array. Then for a Holographic CDMAsystem with K users, where K≦n, each shift can be assigned to adifferent source/user in system 100.

The light signals transmitted through masks 151-153 in FIG. 1,designated as S₁ (x,y), S₂ (x,y), and S_(K) (x,y), respectively, serveas inputs to optical broadcast network 105. Network 105 is arranged toform a composite signal, designated S_(T) (x,y), which has the followingform: ##EQU3## where K is the number of sources/users. Thus S_(T) (x,y)is a linear combination of all the modulated light signals transmittedby masks 151-153.

The arrangement of FIG. 4 depicts an illustrative embodiment for opticalbroadcast network 105 of FIG. 1. Optical signals S₁, S₂, S_(i), andS_(K) (the argument (x,y) for each signal has been dropped for ease ofpresentation), serve as inputs to network 105. S₁ is reflected frommirror 410 onto beam spitter 420. S₂ also impinges on beam splitter 420so that the output from splitter 420 in the downward direction towardsbeam splitter 421 may be expressed as (S₁ +S₂)/2. S_(i), that is, thesignal originating from the i^(th) source (not shown explicity inFIG. 1) and impinging on network 105, passes through attenuator 431 andexcites beam splitter 421 in the horizontal direction. The attenuator isset to 0.5 so that the signal emanating from splitter 421 is thedownward direction towards beam splitter 422 is expressed as (S₁ +S₂+S_(i))/4. Finally, S_(K) is passed through attenuator 432, with itsattenuation value set at 0.25, and impinges on beam splitter 422 alongwith the output of splitter 421. The composite signal emerging fromsplitter 422 in the horizontal direction, which may be represented by(S₁ +S₂ +S_(i) +S_(K))/8, is passed through optical gain device 441. Ifdevice 441 has a gain of 8.0, then the signal emerging from device 441is S_(T) as defined in equation (1).

Again with reference to FIG. 1, composite signal S_(T) (x,y) emerges onK optical paths from network 105. The first output optical path feedsS_(T) (x,y) to Fourier Transform lens 161. Hologram 171, also labeled asS₁ Hologram in FIG. 1, is placed at the focal length distance (F_(L))behind lens 161. The signal transmitted through hologram 171 isintercepted by focusing lens 181 placed in a strategically locatedposition behind hologram 171; the precise placement will be discussedbelow shortly. Lens 181 delivers a demodulated optical signal to fiber131, and in turn, fiber 131 propagates this demodulated optical signalto receiver 111. The combined operation of the cascade of FourierTransform lens 161, hologram 171, and focusing lens 181 is referred toas optical holographic CDMA decoding using an optical matched filter.

Holographic CDMA decoding is obtained by arranging lens 161, hologram171 and lens 181 to implement the optical matched filter; this filtermaximizes the ratio of peak signal energy to rms noise. One realizationof this matched filter is obtained by an optical method introduced by A.Vander Lugt in the article entitled "Signal Detection by Complex SpatialFiltering", as published in the IEEE Transactions of Information Theory,IT 10:2, pp. 139,145, April, 1964. The optical matched filter has atransfer function which is the complex conjugate of the code imagespectrum.

With reference to FIG. 5, there is shown hologram generator arrangement200 for generating each S_(i) Hologram for the s_(i) (x,y) mask, i=1,2,. . . K, of FIG. 1. Arrangement 200 uses reference beam 241 to interferewith the output of Fourier transform lens 260 at hologram 270. Hologram270 is any medium that registers light intensity, such as photographicfilm. Laser source 210, which is illustratively an argon laser operatingat 514.5 nm, illuminates collimating lens 220; in turn, the output oflens 220 is directed to beam splitter 230, with the horizontallytransmitted component impinging on mirror 240 and the verticallydeflected beam being modulated by mask 250 representative of array s_(i)(x,y), i=1, 2, . . . , or K. The angle of light signal 241 reflected bymirror 240 is α. The output light from mask 250 impinges on FourierTransform lens 260. Finally, both the light signal from lens 260 and thereflected light from mirror 240, shown as beam 241, are focused onhologram 270. Arrangement 200 creates the desired intensity pattern onhologram 270 so that when each hologram representative of each uniques_(i) (x,y) mask is embedded in system 100 of FIG. 1, matched filterdetection may be effected.

Again with reference to FIG. 5, if F₁ (p,q) denotes the output of lens260, which displays a light signal which is the Fourier transform of s₁(x,y) at its back focal plane, with p and q representing spatialfrequency, and if R(p,q) represents the light coming from mirror 240,with R(p,q)=|R(p,q)|e^(j)φ(p,q), where |R(p,q)| is a constant and φ(p,q)is linear in phase, then the intensity pattern on the holographicrecording film is, ##EQU4## The fourth term in equation (2) representsthe desired filter function, F_(i) * (p,q), multiplied by the linearphase factor of R(p,q) since |R(p,q)| is constant. Once the matchedfilters, that is, the holograms, for different codes are sequentiallyproduced beforehand by hologram generator 200, the holograms are thenphysically located at the receiving end of system 100, namely, asholograms 171, 172, 173.

The exact placement of, for example, focusing lens 181 relative tohologram 171 in FIG. 1 is depicted in detail in FIG. 6. It can bedemonstrated that the first two terms from equation (2) give rise to alight beam aligned with optical axis 172 of FIG. 6. For purposes of thisinvention, this light signal is ignored. Another transmitted light beamemerges from hologram along optical axis 173 which is offset fromoptical axis 172 by the downwardly directed angle α. This light signalalong axis 173 is the output from the optical matched filter andcorresponds to the fourth term in equation (2). Finally, forcompleteness, the third term in equation (2) corresponds to the beamemerging from hologram 171 along optical axis 174 at an upward angle α,and this beam is also ignored. Complete details for aligning lens 181with hologram 171 may be found in the text "Introduction to FourierOptics", authored by J. W. Goodman, published by McGraw-Hill BookCompany, 1968; particular reference should be made to pages 171-177.

Briefly, by way of an operational description, the component S₁ (x,y)present in S_(T) (x,y) will have a wavefront curvature which will bebrought into focus by Fourier Transform lens 181 to thereby generate abright intensity light signal focused at the input to fiber medium 131;this focusing occurs since S₁ hologram 171 is matched to mask 151, thatis, the s₁ (x,y) mask.

On the other hand, when, for example, light component S₂ (x,y) in thecomposite signal S_(T) (x,y) is incident on the hologram 171, the outputwill have a random-like wavefront curvature which will not be brought toa bright focus by the Fourier Transform lens 181. If it is assumed theproperly decoded signal has a bright spot with intensity one, any othersignal present in the composite signal will have, on average, anintensity ##EQU5## where NM=n is the number of pixels in a mask (code)with N×M dimensions. The large contrast in the intensities between amatched, decoded signal and an unmatched, decoded signal is used todistinguish between correctly and incorrectly addressed signals, thatis, to distinguish among sources.

It is to be understood that the above-described embodiments are simplyillustrative of the application of the principles in accordance with thepresent invention. Other embodiments may be readily devised by thoseskilled in the art which may embody the principles in spirit and scope.Thus, it is to be further understood that the methodology andconcomitant circuitry described herein is not limited to the specificforms shown by way of illustration, but may assume other embodimentslimited by the scope of the appended claims.

What is claimed is:
 1. A code division multiple access system forinterconnecting a plurality of sources to a corresponding plurality ofreceivers, each of the sources including an optical interface to producean optical source signal, the system comprisinga plurality of opticalencoders for generating an associated plurality of encoded opticalsignals, each of said optical encoders being responsive to the opticalsource signal from a corresponding one of the sources, with each of saidoptical encoders including a unique spatial optical encoding mask forgenerating a corresponding one of said encoded optical signals, saidmask having an optical transmission pattern determined from apreselected set of patterns wherein said patterns correspond tosequences with predetermined correlation properties, an opticalbroadcast network, coupled to said optical encoders, for combining saidencoded optical signals to generate a composite optical signal as anoutput of said broadcast network, and a plurality of optical detectors,responsive to said composite signal, for generating an associatedplurality of decoded optical signals, each of said decoded opticalsignals being assigned to one of the receivers, each of said opticaldetectors being arranged to detect a corresponding one of said encodedoptical signals in said composite optical signal, and wherein each ofsaid optical detectors includes a holographic decoder corresponding tosaid unique spatial optical encoding mask used to generate saidcorresponding one of said encoded optical signals, said holographicdecoder producing said each of said decoded optical signals.
 2. Thesystem as recited in claim 1 wherein each of the sources includes aninformation signal and a light source for generating the optical sourcesignal as representative of the information signal, wherein each saidoptical encoder includes a collimating lens for optically spreading theoptical source signal to produce a spatially spread light signal, andwherein said encoding mask includes transparent and opaque regionsarranged according to one of the patterns in said set of patterns, andsaid encoding mask is positioned to modulate said spread light signal tothereby generate said corresponding one of said encoded signals.
 3. Thesystem as recited in claim 1 wherein each of the sources includes aninformation signal and a light source for generating the optical sourcesignal as representative of the information signal, wherein each saidoptical encoder includes a collimating lens for optically spreading theoptical source signal to produce a spatially spread light signal, andwherein said encoding mask includes first regions of transmission havinga first phase shift and second regions of transmission having a secondphase shift, said first and second regions arranged according to one ofthe patterns in said set of patterns, and said encoding mask ispositioned to modulate said spread light signal to thereby generate saidcorresponding one of said encoded signals.
 4. The system as recited inclaim 1 wherein each of said holographic decoders comprisesa FourierTransform lens for filtering said composite signal to produce atransformed optical signal, a hologram corresponding to said uniquespatial optical encoding mask, said hologram being located at the focaldistance from said Fourier Transform lens and serving as an opticalmatched filter to convert said transformed optical signal to aholographic optical signal, and a focusing lens aligned on the matchedfilter axis of said hologram to convert said holographic optical signalto said each of said decoded optical signals.
 5. The system as recitedin claim 1 wherein the sources and the receivers are geographicallydisperse, and wherein the system further comprisesa first plurality ofoptical fibers interconnecting the plurality of sources to saidplurality of optical encoders, and a second plurality of optical fibersinterconnecting said plurality of optical decoders to the plurality ofreceivers.
 6. A system for interconnecting a plurality of incominginformation-bearing light signals to a corresponding plurality ofoutgoing information-bearing light signals, the system comprisinganplurality of encoders, responsive to the incoming light signals, forgenerating a corresponding plurality of modulated light signals, whereineach of said encoders is responsive to a corresponding one of theincoming light signals and includes:an input collimating lens forspatially spreading said one of said incoming light signals to produce aspread light signal; and an encoding mask, optically coupled to saidincoming collimating lens, for modulating said spread light signal toproduce a corresponding one of said modulated light signals, said maskincluding first regions having a first transmission characteristic andsecond regions having a second transmission characteristic wherein thearrangement of said first regions and second regions on said maskcorrespond to a preselected pattern from a set of code patterns, anoptical broadcast network for combining said modulated light signalsinto a composite light signal, a plurality of decoders, coupled to saidbroadcast network, for generating the outgoing optical signals, whereineach of said decoders is assigned to a corresponding one of saidencoders and includes:a Fourier Transform lens for receiving saidcomposite signal to produce a transformed light signal; a hologram,located at the optical focal distance from said Fourier Transform lens,for generating a holographic light signal from said transformed lightsignal, said hologram representative of one of said code patternsassigned to said corresponding one of said encoders; and an outgoingfocusing lens, aligned on the matched filter axis of said hologram, forgenerating a corresponding one of the outgoing light signals from saidholographic light signal.
 7. A method for combined encoding and decodingof an incoming optical signal to produce a outgoing optical signalcomprising the steps ofspreading the incoming optical signal with acollimating lens to produce a spread optical signal, modulating saidspread optical signal with a unique spatial optical encoding mask toproduce an encoded optical signal, said mask having an opticaltransmission pattern determined from a preselected set of patternswherein said patterns correspond to sequences with predeterminedcorrelation properties, filtering said encoded optical signal with aFourier Transform lens to produce a transformed optical signal,modulating said transformed optical signal with a hologram positioned atthe optical distance from said Fourier Transform lens to produce aholographic light signal, said hologram being representative of saidencoding mask, and filtering said holographic light signal with afocusing lens to produce the outgoing optical signal, said focusing lensbeing aligned on the matched filter axis of said hologram.